1. Technical Field
The invention relates to a plasma picture screen provided with a front plate comprising a glass plate on which a dielectric layer and a protective layer are provided, with a carrier plate provided with a phosphor layer, with a ribbed structure which subdivides the space between the front plate and the carrier plate into plasma cells which are filled with a gas, and with one or several electrode arrays on the front plate and the carrier plate for generating electrical corona discharges in the plasma cells.
2. Description
Plasma picture screens render possible color pictures with high resolution and large picture screen diagonals and are of a compact construction. A plasma picture screen comprises a hermetically closed glass cell which is filled with a gas, with electrodes in a grid arrangement. The application of a voltage triggers a gas discharge which generates light in the ultraviolet range. This light can be converted into visible light by means of phosphors and be emitted to the viewer through the front plate of the glass cell.
In plasma picture screens commercially available nowadays, BaMgAl10O17:Eu (BAM) is mostly used as the blue phosphor, Zn2SiO4:Mn (ZSM) as the green phosphor, and (Y,Gd)BO3:Eu (YGB) as the red phosphor. The choice of these phosphors was made mainly on the basis of their luminous efficacy upon excitation by VUV radiation and their color points. The color points of the phosphors used play a very important part in emitting picture screens because they determine the maximum achievable color space.
All of the selected phosphors, however, have certain disadvantages which cannot be easily eliminated. Thus the blue phosphor BaMgAl10O17:Eu shows a strong decrease in brightness when irradiated with VUV. Owing to this degradation, the white color point of the plasma picture screen is shifted towards the yellowish orange, because the green and in particular the red phosphor are degraded less strongly.
The green phosphor Zn2SiO4:Mn has a comparatively long decay time of approximately 10 ms and only a moderately good luminous efficacy when excited by VUV light. The efficacy clearly decreases, moreover, at increasing luminance owing to saturation. In addition, its stability is also less than that of the red phosphor (Y,Gd)BO3:Eu when excited by VUV light.
The red phosphor (Y,Gd)BO3:Eu is indeed very efficient under excitation by VUV light, but its color point is less red than that of Y2O2S:Eu, which is used in cathode ray tubes.
To avoid the disadvantages of the individual phosphors described above, the phosphors are, for example, provided with a coating, or alternative phosphors are synthesized. DE 197 27 607, for example, discloses a blue-emitting aluminate phosphor with a coating of catena-polyphosphate of one or several alkaline earth metals, zinc, Camden, and/or manganese. The coating of catena-polyphosphate achieves that the degradation of the phosphor by the VUV excitation is reduced. Until now, however, no alternative phosphors to the three standard phosphors BaMgAl10O17:Eu, Zn2SiO4:Mn, and (Y,Gd)BO3:Eu which fulfill all the requirements and are capable of replacing said three phosphors could be synthesized.
It is accordingly an object of the present invention to avoid the disadvantages of the present art situation and to make available an improved plasma picture screen.
This object is achieved by means of a plasma picture screen provided with a front plate comprising a glass plate on which a dielectric layer and a protective layer are provided, with a carrier plate provided with a phosphor layer comprising a mixed particle mixture of at least two phosphors which emit the same color, with a ribbed structure which subdivides the space between the front plate and the carrier plate into plasma cells which are filled with a gas, and with one or several electrode arrays on the front plate and the carrier plate for generating electrical corona discharges in the plasma cells.
The use of two phosphors which emit the same color in one and the same phosphor layer makes it possible to reduce or mutually compensate the undesirable properties of the phosphors.
It is preferred that the phosphor layer is subdivided into individual color segments, and at least one segment comprises a mixed particle mixture of at least two phosphors which emit the same color.
The phosphor layer of a plasma picture screen usually comprises several color segments, each with its own phosphor which emits in the color blue, green, or red. The use of two different phosphors which emit the same color in one color segment is capable of improving the properties of this color segment.
It is particularly preferred that a color segment of the phosphor layer comprises a mixed particle mixture of BaMgAl10O17:Eu and a phosphor chosed from the group of Ce3+-activated phosphors and Tm3+-activated phosphors.
The luminous decrement of a blue-emitting phosphor and of a blue-emitting color segment of a phosphor layer during the operational life of a plasma picture screen can be lessened because Ce3+-activated phosphors and Tm3+-activated phosphors have a higher stability under irradiation with VUV light than BaMgAl10O17:Eu.
It is particularly highly preferred that the Ce3+-activated phosphor is chosen from the group of YBO3:Ce and (Y1-xGdx)BO3:Ce, with 0xe2x89xa6xxe2x89xa61.
It is also particularly preferred that the Tm3+-activated phosphor is LaBO3:Tm.
The color point of a blue-emitting color segment of a phosphor layer may be further improved because YBO3:Ce, (Y1-xGdx)BO3:Ce with 0xe2x89xa6xxe2x89xa61 and LaBO3:Tm have a deeper blue emission color than BaMgAl10O17:Eu.
It is furthermore preferred that a color segment of the phosphor layer comprises a mixed particle mixture of Zn2SiO4:Mn and a Tb3+-activated phosphor.
The luminous decrement of a green-emitting color segment of a phosphor layer during the operational life of a plasma picture screen can be lessened because Tb3+-activated phosphors have a higher stability than Zn2SiO4:Mn under irradiation with VUV light. This is because Tb3+ can be oxidized to Tb4+ with difficulty only, whereas Mn2+ is very prone to oxidation. The luminous efficacy of a green-emitting color segment of a phosphor layer, moreover, can be enhanced, especially at higher luminance levels. In addition, the color brilliance of a green-emitting color segment is improved owing to a lower saturation.
It is advantageous when the Tb3+-activated phosphor is chosen from the group of LaPO4:Ce,Tb, Y2SiO5:Tb, GdMgB5O10:Ce,Tb, CeMgAl11O19:Tb, GdBO3:Tb, (Y1-xGdx)BO3:Tb, with 0xe2x89xa6xxe2x89xa61, YBO3:Tb, LaOCl:Tb and InBO3:Tb.
These Tb3+-activated phosphors have a shorter decay time than Zn2SiO4:Mn. Motion artefacts in the picture composition can be reduced by means of a mixed particle mixture of these Tb3+-activated phosphors with Zn2SiO4:Mn in a green-emitting color segment of a phosphor layer.
It may be preferable, in addition, that a color segment of the phosphor layer comprises a mixed particle mixture of (Y,Gd)BO3:Eu and a second red-emitting phosphor whose color point (x,y) has a y-value  less than 0.36.
The color point of a red-emitting color segment of a phosphor layer can be improved through the use of a mixed particle mixture of (Y,Gd)BO3:Eu and a phosphor whose emission color lies less in the orange, and more in the red region of the color triangle.
It is advantageous when the second red-emitting phosphor is chosen from the group of Y2O3:Eu, YVO4:Eu, Y(V,P)O4:Eu and (Y1-xGdx)2O3:Eu, with 0xe2x89xa6xxe2x89xa61.
These Eu3+-activated phosphors have a shorter decay time than (Y,Gd)BO3:Eu, whereby also motion artefacts in the picture composition can be reduced.
The invention will be explained in more detail below with reference to a drawing and an embodiment.